Часть 2 (1121000), страница 69
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They too will evolve at a ratethat depends not only on the frequency of genetic or epigenetic changes, butalso on the local influences affecting the survival, growth, proliferation, andspread of these altered cells.Contributeto a SignificantVirusesand OtherInfectionsProportionof HumanCancersA small but significant proportion of human cancers,perhaps 15% in the worldas a whole, are thought to arise by mechanisms that involve viruses, bacteria, orparasites.The main culprits, as shor,m in Table 2O-1, ate the DNA viruses. Evidence for their involvement comes partly from the detection of viruses in cancerpatients and partly from epidemiology.
Liver cancer,for example, is common inparts of the world (Africa and SoutheastAsia) where hepatitis-B viral infectionsn o r m a lc e l l s|'r'rrnro*i s o l a t e dc e l l h a sm u t a t i o n b u tits growth is restrainedI eno'vort^RELEAsESREsrRArNrsImutantcellsgrowintoa largeclonein whicha f urthermutationof cells,mayoccurFigure20-24 The effect of a tumorpromoter.Thetumor promotercreatesalocalenvinrnmentthat expandsthepopulatiorrof mutantcells,therebythe probabilityof tumorincreasingprogressionby furthergeneticchange'1228Chapter20:CancerTable20-1 VirusesAssociatedwith HumanCancersPapovavirusfamilvPapillomavirus(manydistinctstrains)HepadnavirusfamilyHepatitis-BvirusHepatitis-CvirusHerpesvirusfamilyEpstein-Barrviruswarts (benign)carcinomaof the uterinecervtxworldwideworldwidelivercancer(hepatocellularcarcinoma)livercancer(hepatocellularcarcinoma)SoutheastAsia,tropicalAfricaworldwideBurkitt'slymphoma (cancerof B lymphocytes)nasopharyngealcarcinomaWestAfrica,PapuaNewGuineaSouthernChina,GreenlandRetrovirusfamilyHumanT-celladultT-cellleukemia/leukemiaviruslymphomatype | (HTLV-I)HumanimmunoKaposi'ssarcomadeficiencyvirus(Hlv the AIDSvirus)Japan,WestlndiesCentralandSouthernAfricaForalltheseviruses,th numberof peopleinfectedismuchlargerthanthe numberswho developcancer:the virusesmu act in conjunctionwith otherfactors.Moreover,someof the virusescontributeto canceronlyindirectly;HlV,for example,destroyshelperT lymphocytewhichallowsa herpesvirusto transformendothelialcells.Similarly,hepatitis-cviruscauseschron hepatitis,whichpromotesthe developmentof livercancer.are common, and in those regions the cancer occurs almost exclusively in people who show signs of chronic hepatitis-B infection.
chronic infection with hepatitis-c virus, which has infected 170 million people worldwide, is also clearlyassociatedwith the development of liver cancer.The precise role of a cancer-associated virus is often hard to decipherbecause there is a delay of many years from the initial viral infection to thedevelopment of the cancer. Moreover, the virus is responsible for only one of aseriesof steps in the progression to cancer,and other environmental factors andIn some cancers,viruses seem to have additional, indirect tumor-promotingactions.
The hepatitis-B and c viruses may, for example, favor the developmeniof liver cancer by causing chronic inflammation (hepatitis), which stimulatescell division in the liver, and enhances the viruses' more direct effects on cellpylori, which causesulcers, appears to be a major cause of stomach cancer.Andbladder cancer in some parts of the world is associated with chronic infection bythe blood fluke, Schistosoma haematobium, a parasitic flatworm.( ssard^]rsra^runproJXO[!oJJuorssil.uradqllM't/6 t 'zv-ta:09'lsultDuD)'uoqewrew'B/lDN'r'uMor8r pue alol drauv '816t 'ueuroarl'H M :o)sr)uerlu e S ' ^ l a r ) ops u ea ) u a r ) S: r a ) u e J ' s u l r p J ' fuorj) ]a)ue) lsearqJo luaurdola^apatourordue)'ua60rlsaI1;eoadsa'sauouiloqa^rlrnporoar suorleurqurolJourelra)ol arnsodxaleLllMatAaqluoddnsoslea)uoprAale)r60lorL!aprdaJosaurlraqlo leranas'sauouroLlo] sosuodsal'lseatqluanbasqnsrreqg6uue1;eaql Jo sllo)aql Jo uorlPrlualajJrpJo atelsaql ur abueq)luaueurad e ur 1;nsar{euI>ueu6erdural-llnJlslUeql teq] rq6noqrs rl l ) s u a q t r a l e a r Oo q l ' p l r q )] s r Ua q l;o qlrlg ol dn sauouloq o^tl)npordalo]ernsodxa1opouadaqt loOuolaqf ueLuoMssalplrq)p roJleql oi o^rtelaiIir;rqeqordaqt sMoqsqderbaql plrq)lsrUrall or qlrlqsanr6aqs q)rqM]e a6eaq1;ouorl)unJe sPpailolo sreJrls,ueuroMe ur aurl aurosra)ue) lsearq1o,{lrlrqeqord1e6urdo;anapa^rlPlaraql'la)ue) lseaJq]o Ist.taqluo 6ur.reaqp;rq)ro sl)arrf 97-97 arn6r3plrql]sr] spqueuJoMqtrqnn1ea6e:aszsloLvdla@oatt'oraraa90nllfolaorotol'oo0t5.a- _ -UAtIlO/\^.ssalpllqrafaaalvtaa('I1ar>o5ra)ue) ue)uaurvtrtolJuorsstu.uadqll471'5002'sarn6Upup sl)pl la)ue) uolJpaldepv) aberaneuo'ra6uolanr;a;doadsesaleJra)ue) ur aseal)utalqe]t^aulaq] roj alesuaouro)o] papaauareasoqtelrl elep patsnfpe-a6y'lueulearlpue uorl)olap ut sluJr..uJ^oloultolpuodsoro){eu satelqleaplalue) raq}oul suorl)npar]ua)au ratrDgo)tlaHqtllr'\uorlfa1urJo suloDpdur lo letp ut sa6ueqlo] palelarIlqrssod'latue)q)eurolsruotJsLllpapur lleJaq] pue '6ur1ousol)pqol'saxasqloqJo uratled alll 6ulMolloJroJra)ue) 6un; ur asrr)tleuretpaq] aloN'saleLu(B)puesalpuraJ(v)rol palrold are' 0 L 6 1u t u o l l e l n d o d a q r u o l l n q ! 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